1. Priority Information
This application claims priority from European Patent Application No. 03254266.4, filed Jul. 4, 2003, the contents herein incorporated by reference in its entirety.
2. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
3. Description of the Related Art
Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may be used to generate a desired circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist).
The term “patterning device” as here employed should be broadly interpreted as referring to a device that can be used to impart an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning devices include:                a mask: the concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmission mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table/holder/holder, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        a programmable mirror array: one example of such a device is a matrix-addressable surface having a visco-elastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as non-diffracted light. Using an appropriate filter, the non-diffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation mechanism. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described here above, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        a programmable LCD array: an example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table/holder/holder; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth here above.
In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table/holder/holder, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper.
In an alternative apparatus-commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table/holder/holder parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table/holder is scanned will be a factor M times that at which the mask table/holder is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
During the manufacturing process, if the substrate table/holder(s) and/or mask table/holder(s) are moved by an actuator attached to the base frame, a reaction force will be exerted on the base frame. The forces involved can be large because the mass of the table is between 10 and 150 Kg, typically 40–70 kg and the accelerations of the order of 5–100 m/s2. Ideally the base frame is internally very stiff and would be stiffly connected to the floor, but in reality such connections have some compliance. Therefore, the forces can induce vibrations in the base frame which adversely affect the accuracy of the lithographic process, because they disturb the position sensors of the substrate and/or mask table.
To overcome this problem, a system of balance masses, such as that illustrated in FIG. 2, has been proposed. The base frame BF is stiffly connected to the floor 2 via connecting members 4. A substrate table/holder comprising a first element 6 and a second element 7 is supported on the base frame BF. A short stroke actuator 8 connects the first element 6 and the second element 7. The base frame BF also supports a balance mass 12 connected to the second element 7 of the substrate table by long stroke actuator 10.
To move the substrate that is situated on the second element 7, the short stroke actuator 8 is activated and generates a force to move the element 7. Simultaneously, the first element 6 is moved by the long stroke actuator 10 in such a way the short stroke actuator 8 remains operational because the relative position of first element 6 to second element 7 is kept constant. So the first element 6 keeps tracking the position of the second element 7. This is a so-called “slave position control loop”. Applying this principle, the short stroke actuator 8 can be designed to be very accurate over a small working range (short stroke). The long stroke actuator 10 can be designed to be less accurate but it can operate on a large stroke (long stroke). The reaction force from actuator 10 is applied to the balance mass 12 that is left free to move in the actuated direction. By this, the reaction force cannot disturb the base frame.
Therefore, the net force exerted on the base frame is zero. The distance the substrate table WT and the balance mass 12 move relative to each other is determined by the ratio of their masses, according the law of conservation of momentum. Such a system is discussed in U.S. Pat. No. 5,815,246. U.S. Pat. No. 5,969,441 discusses a similar system where a common balance mass is used for two object holders.
Various alternative systems of balance masses are known. For example in one such alternative system the balance mass described above may take the form of a sub-frame that supports the substrate table (that may consist of 1 or 2 elements). An actuator connects the sub-frame to the substrate table. The sub-frame is free to move relative to the base frame. When the actuator is operated, the sub-frame moves under the reaction force, as described above, but no vibrations are transferred to the base frame. A damping system can connect the sub-frame (balance mass) to the base frame to reduce the range of movement of the sub-frame.
In all the above systems, the balance mass is connected to the substrate table by an actuator. While these solutions reduce the forces transmitted to the base frame and the consequential vibrations of the base frame, the complexity of the systems introduces further problems. These systems are also difficult to adapt for use when the substrate table is moved with six degrees of freedom all of which generate large acceleration forces because the balance mass must then also be capable of six degrees of freedom.